You may want to look at the plug itself. It has a locking turn tab device that secures the wire harness .
Slight chance they did not turn the locker when plugging in the harness and it has unplugged.
Remote, but possible....

Also , if they swapped out the tranny w/used one, likely you got the used sw instead of your original one.
These switches are susceptical to water due to there location.
so usually with a tranny swap , it is mileage time for a new one
anyway..
When they go , you are STUCK.....so cheap insurance is new one
while the trans is out....